1. Field of the Invention
This invention relates to a surface treatment method for quartz material to improve heat resistance of quartz material such as quartz glass and prevent diffusion of impurities into the quartz material.
2. Description of Related Art
Generally, quartz material represented by quartz glass, can be easily purified in high concentration and have a high heat resistance, and thus this material has been widely used for various kinds of jigs and structural material for producing semiconductors. In addition, semiconductors have been recently promoted to be integrated, and the promotion of the integration causes diffusion of impurities contained in quartz glass, etc. not to be negligible. Therefore, still higher purity has been required for the quartz material.
On the other hand, it is general that the heat-resisting temperature of quartz glass or the like decreases as its purity increases. Therefore, when it is used as a jig or an element for the semiconductor manufacturing, not only the purity is required to be improved, but also the heat resistance is required to be remarkably promoted.
To improve the heat resistance and prevent the diffusion of impurities for high-purity quartz glass, etc., there has been conventionally proposed a method in which the surface of quartz glass or the like is nitrided, or a method in which a heat-resisting layer such as boron nitride (BN), aluminum nitride (AlN), silicon nitride (Si.sub.3 N.sub.4) or the like is formed. For example, as proposed in Japanese Laid-open Patent Application Nos. 60-246281 and 4-59633, quartz material is disposed in an ammonia atmosphere to nitride the surface of the quartz material. Further, as proposed in Japanese Laid-open Patent Application No. 59-227800, a surface treatment technique such as a plasma CVD method or the like is used to form a BN or AlN coating layer on the surface of the quartz material. Still further, Japanese Laid-open Patent Application No. 62-96349 proposes a quartz glass reaction tube for a semiconductor heat treatment, on the outer surface of which a minute silicon nitride layer is formed by a CVD method of flowing silicon tetrachloride (SiCl.sub.4) and ammonia gas (NH.sub.3) at a high temperature of 1200.degree. C. under a vacuum condition.
However, the conventional methods as described above have the following disadvantages.
In the first method of nitriding the surface of the quartz material with ammonia as disclosed in Japanese Laid-open Patent Application No. 60-246281, a nitriding rate at a temperature below 1200.degree. C. is low, and substantially no nitriding process progresses in this temperature range. Accordingly, in this method the quartz material is required to be subjected to the nitriding treatment at a higher temperature of 1200.degree. C. for example. However, in the high temperature range above 1200.degree. C., the quartz material is liable to be thermally deformed.
In the second method as disclosed in Japanese Laid-open Patent Application No. 4-59633, porous silica is nitrided with ammonia at a temperature below 1000.degree. C. However, this method is applicable to only soot-like fine particles of silica, and thus it is impossible that after molding the silica fine particles into an article such as a semiconductor manufacturing jig, the nitriding treatment using ammonia at 1000.degree. C. or less is conducted on the article.
On the other hand, in the third method of forming the BN or AlN coating layer of high heat resistance on the surface of the quartz material serving as a base member, when the quartz material and the coating layer are subjected to a heat cycle which is usually used in a semiconductor manufacturing process and repetitively conducted in a broad temperature range from a room temperature to 1000.degree. C. or more, there is a possibility that the coating layer exfoliates from the quartz material because there is a large difference in thermal expansion coefficients between the quartz material and the heat-resisting coating layer. In order to prevent the exfoliation of the coating layer from the qartz material, an intermediate layer of Si.sub.3 N.sub.4, a silicon carbide (SiC) or AlN is formed between the quartz material (quartz glass or the like) and the BN or AlN coating layer as disclosed in Japanese Laid-open Patent Application No. 59-227800. The material used for the intermediate layer has a thermal expansion coefficient which locates between the thermal expansion coefficients of the BN or AlN coating layer and the quartz glass, however, the difference in thermal expansion coefficients between the quartz material and the intermediate layer is still large, so that the exfoliation problem cannot be perfectly solved.
Still further, in the method as disclosed in Japanese Laid-open Patent Application No. 62-96349, the surface treatment is required to be conducted above 1200.degree. C. and thus the thermal deformation may occur in the treatment process. In addition, with respect to the minute silicon nitride layer formed on the outer surface, the exfoliation problem based on the difference in thermal expansion coefficients between the quartz and the silicon nitride layer also occurs.